The present invention generally relates to rotating machinery, such as gas turbine engines, and more specifically, to piloting a nut used on a shaft to apply a compressive axial force to a plurality of stacked components to position the components and to position the nut on the shaft.
In rotating assemblies used in high speed machinery, the components are often clamped either by a tie-shaft and nut or by bolted flange joints. In many applications, nuts and bolts are used to apply compressive forces on multiple components, securing them in a stacked relationship. The compressive force through the components is equal to the tensile force in the bolt(s), which stretches proportionally to the bolt length. These nuts and bolts maintain the axial location of the components relative to each other and must also ensure that radial position is controlled.
Gas turbine engines include rotating components such as a fan, a compressor, a shaft, a seal and a turbine. A nut is often used on the end of a threaded shaft to secure and position engine components relative to the shaft. The shaft typically has a radial flange extending outward at one end to provide an abutting surface and threads for the nut at the opposite end. The engine components are stacked along the shaft such that the shaft extends through the center of the components. The nut is threaded to the shaft to apply a compressive force through the components that secures them in place relative to the shaft, and thus pilots the components.
Components in a rotating group require an axial facing pilot and a radially oriented pilot when mated to another component. Components that are located between two other components require an axial facing pilot and a radially oriented pilot at each interface. The threads of a nut and bolt (or tie-shaft) provide both an axial facing pilot and a radially oriented pilot at the nut to tie-shaft interface. However, at the nut to rotor stack interface, often only an axial pilot is provided.
The axial facing pilot and radially oriented pilot require geometric control such that these features are true to each other (perpendicular). Lack of perpendicularity of the axial facing pilot and radially oriented pilot results in shaft bow. It is easy to control the perpendicularity between the face and diameter of a component, however, it is difficult to have precision control between the threads of a nut and the face of the nut. This is also true of a bolt, tie-shaft, or other threaded component(s).
When a tie-shaft and nut are used, problems often occur, such as problems with balance repeatability and associated vibration effects due to a lack of piloting of the nut, or shifting of the nut relative to the rotor stack due to lack of radial piloting of the nut. Various conventional designs for the tie-shaft and nut have been proposed and used in gas turbine engines to maintain position control of the nut relative to the rotor stack.
One such conventional design is disclosed in U.S. Pat. No. 5,022,823 to Edelmayer (“Edelmayer patent”). FIG. 1 shows a prior art rotor attachment assembly 10 for securing a rotor 12, such as a compressor impeller, to a rotor shaft 14, generally according to the Edelmayer patent. The shaft 14comprises a smooth shaft body 24 and a threaded nut-receiving portion 26, which may have a smaller diameter than the shaft body 24. The nut 16 includes an unthreaded shaft locating hole 30 and a threaded hole 28. When the nut 16is fully threaded onto the shaft 14, a nut mating surface 20 of the rotor 12 and the rotor mating surface 32 of the nut 16 mate to create an axial load across the rotor 12 to axially secure the rotor 12 with the shaft 14. The unthreaded shaft-locating hole 30 provides a radial pilot of the nut 16 relative to the shaft body 24. This feature of the Edelmayer patent provides a positive radial pilot for the nut 16 to shaft 14.
Again with reference to the prior art assembly of FIG. 1, when the nut 16 is tightened onto the shaft 14 to press against the rotor 12, an axial load is left between the body 24 of the shaft 14 and the threaded hole 28 of the nut 16. Furthermore, as the nut 16 is tightened, the unthreaded shaft locating hole 30 may expand outwardly, reducing the fit between unthreaded shaft-locating hole 30 and the shaft body 24, allowing the nut 16 to move radially relative to the shaft 14. This may result in a loss of nut radial piloting to shaft 14 and an increase in rotor bow and unbalance. The Edelmayer design requires very close tolerances between the shaft 14 and the nut 16 to assure coaxiality of the shaft 14 and nut 16 to minimize shaft bending. The tolerances of Edelmayer are so close so as to preferably comprise an interference fit between the unthreaded shaft locating hole 30 and the body 24 of the shaft 14, which makes tightening of the nut 16 difficult. Unfortunately, obtaining and maintaining the close tolerances involved in the Edelmayer patent requires considerable labor and expense.
As can be seen, there is a need for an improved apparatus and method for maintaining group balance, including balance repeatability when a rotating group is secured with a nut and tie-shaft or like axial loading feature. Furthermore, there is a need for an improved apparatus and method that does not require extremely close tolerances or an interference fit of the nut to the shaft.